The present invention relates to an image recording apparatus for use in copying machines, printers, and facsimiles, etc.
In the image recording apparatus of this kind, electric potential is applied to a selected one of the plurality of apertures of a toner flow control means such as an aperture electrode body in accordance with image data, so that the charged toners pass through the aperture to form a toner image onto an image receiving member running on an opposing electrode. More specifically, a toner supply unit such as a toner carrier roller is disposed below the aperture electrode body for transferring charged toners to the aperture electrode body, and the opposing electrode is provided above the aperture electrode body for directing the toners passing through the aperture toward the image receiving member. The aperture electrode body includes an insulative substrate made of polyimide in which a plurality of apertures are formed and electrically conductive control electrodes formed of copper which are positioned on the insulative substrate for surrounding respective apertures.
However, such a conventional image recording apparatus, since the toners are firmly fixed to the outer peripheral surface of the toner carrier roller due to image force, and therefore, the toners are not smoothly introduced into the aperture electrodes, i.e., toner flow control by the toner flow control means cannot be sufficiently performed.
To avoid this problem, a laid open European Patent Application publication No. 587,366 discloses an image recording apparatus in which the aperture electrode body is in contact with the toner carrier roller at least at the area of apertures. Still however, another unsolved problem remains.
As shown in FIGS. 1 through 3, in the conventional image recording apparatus, particular areas of the insulative substrate of the aperture electrode body may be deformed due to difference in thermal expansion coefficient relative to the control electrodes. The particular area may be the area on which the control electrode is mounted. The aperture electrode body 71 shown in FIG. 1 includes an insulative substrate 72 and control electrodes 73. The insulative substrate 72 is formed with a plurality of apertures 74 arrayed in one direction and spaced away from each other by a constant distance, and each control electrode 73 surrounds each aperture 74. The insulative substrate 72 and the control electrodes are formed of polyimide and copper, respectively. Here, the thermal expansion coefficient of polyimide is 2.0.times.10.sup.-5 1/.degree.C. whereas the thermal expansion coefficient of copper is 1.7.times.10.sup.-5 2/.degree.C. Copper electrodes are formed on the polyimide substrate by sputtering or plating which require relatively high temperature, Therefore, after sputtering or plating, the shrinkage ratio of the polyimide is greater than that of the copper. As a result, the polyimide substrate is subjected to tensile stress whereas the copper electrodes are subjected to compressive stress in the room temperature.
Accordingly, the aperture electrode body 71 is arcuately deformed as shown in FIG. 2 in such a manner that the electrode body 71 is recessed at the side confronting the toner carrier roller 11. As a result, wrinkle-like inflection curve is generated at the boundary 77 of the control electrode 73 as shown in FIG. 2, where toner particles may be accumulated or deposited, to degrade toner supply to the aperture 74, to thus reduce imaging density.
Further, as shown in FIGS. 1 and 3, with respect to the direction of the array of the apertures 74, the aperture electrode providing portion (zone X) is recessedly deformed with respect to the surface of the toner carrier roller 11, whereas the aperture electrode non-providing portion (zone Y) is maintained flat. Therefore, the zone Y is in pressure contact with the toner carrier roller 11, whereas the zone X is floated over the roller 11. Consequently, at the boundary portion 78 between the zones X and Y, toners are forcibly supplied into the aperture 74 positioned at the extreme end of the aperture array. Accordingly, imaging density at the extreme portion becomes high, and fog may occur in the resultant image.